1. Field of the Invention
The present invention relates to a paper calendering apparatus. Specifically, it relates to a paper calendering apparatus used to improve the surface quality, such as smoothness and gloss, of paper sheets.
2. Description of the Related Art
There exist a variety of types of calendering apparatuses used in the making of paper, typical examples of which are hard nip calenders and supercalenders.
Chilled nip calenders are apparatuses which may be adapted for online finishing as an addition to a paper machine after the drier, whereby the surface quality of paper sheets is modified as they pass through a pair of roller nips, while the surfaces of the metal rollers are chilled.
Supercalenders, on the other hand, comprise a series of alternating resilient rollers and chilled rollers in a vertical direction, and unlike hard nip calenders, the paper sheet rollers are subjected to the high-pressure multinip finishing while offline, due to restrictions on the finishing speed placed in consideration of the life of the resilient rollers, and therefore this type of calender is suited for the production of highly smooth, high gloss paper sheets such as gravure printing sheets.
In addition to the apparatuses described above, recent years have also seen the development of high-temperature soft nip calenders used online in the same manner as hard nip calenders, which aim at extending the life of the resilient rollers by using a resilient roller and a chilled roller as one pair and limiting the number of nips around the resilient roller to one, and high-temperature soft nip calenders which perform high-temperature finishing of paper sheets and guarantee a level of near-supercalender quality in an online manner by heating of the chilled rollers.
In hard nip calenders made of metal rollers, and supercalenders and soft nip calenders which employ resilient rollers, there are clear differences in the basic functions for modifying the surface quality of paper. Freshly dried paper by a paper machine is of uneven thickness, but the following may be said regarding the changes which occur in the surface condition of a paper sheet when passed through the nips of the calender apparatuses described above for finishing, based on a cross section taken through the path of the paper.
First, in the case of a hard nip calender which forms nips with chilled metal rollers, the raised sections of the paper sheet surface are pressed down and made flat, but the depressed sections receive no pressure even when the chilled rollers contact the surface, and this tends to create an uneven gloss, while the density cannot be made uniform despite the uniform thickness of the paper sheet, thus resulting in uneven density.
Next, in the case of a soft nip calender which forms a nip with a resilient roller and a chilled roller, when a paper sheet with non-uniform thickness immediately after drying in a paper machine passes through the calender, the surface of the paper sheet contacting the chilled roller surface is made flat by the smooth surface of the roller. However, during the flat finishing of the chilled roller side, on the rear side of the paper sheet which is the paper sheet surface on the resilient roller side, there appears a more complex unevenness due to the added unevenness from the chilled roller side in addition to its original unevenness. Nevertheless, since the resilient rollers, being resilient, are capable of being deformed by the shape corresponding to the unevenness, the unevenness of the paper surface can also undergo pressure finishing. Also, the density of the paper sheet becomes uniform despite the non-uniformity of the thickness, and the smoothness of the roller surface is transferred to the paper sheet surface on the chilled roller side, thus imparting smoothness and gloss thereto.
When soft nip calendered products and hard nip calendered products are compared in terms of printing suitability and printing surface feel, it is found that the uniform-density soft nip calendered products have uniform absorption and adhesion of ink, while in terms of the bulk, i.e., the specific volume, they also have larger thicknesses than hard nip products as a result of the use of the resilient rollers.
Furthermore, supercalenders perform multinip finishing with a series of alternating resilient rollers (fiber coils) consisting chiefly of cotton, paper and other natural fibers and chilled rollers in a vertical direction, and they are suitable for the production of highly smooth paper such as that required for gravure printing.
However, since in supercalenders the nips are formed with the top and bottom of the fiber rollers in contact with the metal rollers, double linear pressure is undergone with each turn of the fiber rollers, and therefore the fiber rollers, having a relatively low hardness of 75-85 in terms of Shore durometer hardness, are able to ensure a more uniform density of the paper sheet; however, this is not without a considerable degree of elastic deformation at the locations receiving the linear pressure, and thus because of repeated linear pressure within a short period of time, troubles tend to occur including damage by internal heat due to hysteresis, making it impossible to recover the original form.
For this reason, supercalenders are slower than the speed of paper machines of reducing the paper stock and therefore they are provided offline; still, the same problems remain of roller replacement and management as a result of roller damage.
Resilient rollers used in soft nip calenders are constructed with a heat-resistant synthetic resin layer over the full width and circumference of a metal roller surface, and the thickness of the synthetic resin layer is about 10 mm for the purpose of heat release, while the hardness of the resin roller is 85-95 in terms of Shore durometer hardness, which is somewhat higher than the hardness of natural fiber resilient rollers used in supercalenders, and therefore there is less resilient deformity at the nip sections; furthermore, since the resin roller is limited to forming a nip with a metal roller at only one location on its circumference, time is ensured for restoration of the original form after resilient deformity at the nip, the life of the resin layer of the resilient roller is extended, and the calender may be operated online.
However, although soft nip calendered products have better surface quality, including gloss and smoothness, than hard nip calendered products, the nip finishing frequency is lower, and furthermore since the hardness of the resin roller is higher than natural fiber rollers, the surface quality of the paper sheets does not begin to approach that of supercalendered products.
Recently, in order to attain supercalender quality with the above-mentioned soft nip calenders, high-temperature soft nip calendering has been developed wherein the finishing is performed with the metal rollers heated to a high temperature of about 175.degree. C. at which the fibers of the paper sheet begin to deform; this, however, tends to further shorten the life of the resin rollers.
Despite advances in the development of heat-resistant resins their present limit is around 110.degree.-150.degree. C., and therefore currently paper sheets and resin roller surfaces must be monitored while the resin roller surfaces are cooled with cold air, and at temperatures of the cut paper and resin roller surface above the acceptable range the operation must be carried out with an apparatus which allows prompt release of the nips, with the greatest care to damage prevention and general upkeep of the resin rollers.